It is known that the presence of fog is a major danger on the roads. For this reason, in the prior art it has already been proposed to use lighting and/or indicating systems which in particular enable visibility to be improved under foggy conditions (as a lighting function), and which also improve the effectiveness of the various indicators of the vehicle when it is in conditions of poor visibility in relation to other vehicles. In this connection fog lamps have been developed which have optical characteristics whereby the light beams which they emit can penetrate fog more effectively than ordinary lighting and/or indicating devices.
In the prior art, the illumination (lighting) or extinction of the fog lamp is controlled manually by the driver, using a switch which is arranged on the fascia. The signal initiated by the driver in this way is processed by a central lighting and indicating control unit which controls the indicators and lights of the vehicle. When used in the absence of poor visibility, such an arrangement can be a nuisance for the users of other vehicles, and more particularly when the device concerned is a fog lamp. In addition, if the lighting and/or indicating apparatus used in poor visibility has been ignored when there is a need for its use, there is a dangerous situation.
That is why in the prior art there have been proposals for automatic systems such that a fog lamp can be lit or extinguished in response to weather conditions. In particular, optical systems have in the past been proposed which comprise an emitter/receiver for infra-red radiation of suitable wavelength, which is diffused by the fog particles. The part of this radiation which is retrodiffused (diffused back towards the vehicle) is detected, and is then compared with a threshold value in such a way as to cause the fog lamp to be lit or extinguished.
However, poor visibility results from a very varied range of physical conditions, and the above mentioned solutions do not enable all types of poor visibility situations to be detected easily under all circumstances.